gc: make 'invalid rune in string' a little less cryptic
gc: make 'invalid rune in string' a little less cryptic

Fixes issue 371.

R=ken2
http://codereview.appspot.com/164091
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go